AsyncSeq.groupBy, AsyncSeqSrc, docs

[eulerfx]

Adding AsyncSeq.groupBy and a few accompanying functions, particularly AsyncSeqSrc.

The former allows workflows such as the following:

source
|> AsyncSeq.groupBy (fun item -> item.id % 4)
|> AsyncSeq.mapAsyncParallel (
  snd >> AsyncSeq.bufferByCountAndTime 100 100 >> AsyncSeq.iter action)
|> AsyncSeq.iter ignore

The above workflow buffers elements in a group and processes groups in parallel. The implementation of AsyncSeq.groupBy uses AsyncSeqSrc which is an imperative source of async sequences (borrowed from Hopac).

Examples are in the tests project.

[eulerfx]

@dsyme, if you've no objections, I will merge this and do a beta release.